1. Field of the Invention
The present invention relates to a radiographic image pickup apparatus and, more particularly, to a radiographic image pickup apparatus used for medical diagnosis or nondestructive inspection in an industrial process.
In this specification, radiation includes electromagnetic waves such as X-rays, alpha rays, beta rays, and gamma rays, and descriptions will be made based thereon.
2. Related Background Art
X-ray photographing systems installed in hospitals, etc., are divided into analog systems in which a subject is irradiated with X-rays and a film is exposed to X-rays reaching the film after passing through the subject, and digital systems in which X-rays passing through a subject are converted into an electric signal, which is stored, for example.
As a digital system, a radiographic image pickup apparatus is known which is constituted by a phosphor for converting X-rays into visible light and a photoelectric converter for converting the visible light into an electric signal. X-rays pass through a subject and the phosphor is irradiated with the X-rays and converts the X-rays into visible light for information about internal portions of the subject""s body. The photoelectric converter converts the visible light into an electrical signal and outputs this signal. In the form of the converted electrical signal, X-ray image information to be recorded, displayed, printed or used for diagnosis can be treated as digital values after being digitized by an A/D converter.
Radiographic image pickup apparatuses using an amorphous silicon semiconductor thin film for a photoelectric converter have recently been put into practical use.
FIG. 13 is a top view of an example of a photoelectric conversion substrate in which photoelectric conversion devices of a metal insulator semiconductor (MIS) type and switching devices are formed by using an amorphous silicon semiconductor thin film as materials therefor. Wirings for connecting the devices are also illustrated in FIG. 13. FIG. 14 is a cross-sectional view taken along the line 14xe2x80x9414 of FIG. 13. The MIS-type photoelectric conversion device will be referred to simply as xe2x80x9cphotoelectric conversion devicexe2x80x9d in the following description for the sake of simplicity.
Photoelectric conversion devices 301 and switching devices 302 (amorphous silicon TFTs, hereinafter referred to simply as xe2x80x9cTFTxe2x80x9d) are formed on one substrate 303. A lower electrode of each photoelectric conversion device and a lower electrode (gate electrode) of each TFT are formed from a common layer, i.e., a first metallic thin film layer 304. An upper electrode of each photoelectric conversion device and upper electrodes (source electrode and drain electrode) of each TFT are also formed from a common layer, i.e., a second metallic thin film layer 305. Gate drive wirings 306 and matrix signal wirings 307 in a photoelectric conversion circuit section are also formed from the first and second metallic thin film layers. A layer 313 is an N+-layer, a layer 312 is an intrinsic semiconductor layer, and a layer 311 is an insulating layer made of SiNx for example. The pixels in number corresponding to 2xe2x80x22, i.e., four pixels in total are illustrated in FIG. 13. Hatched areas in FIG. 13 represent light receiving surfaces of the photoelectric conversion devices. Power supply lines 309 for biasing the photoelectric conversion devices are also provided. The photoelectric conversion devices and TFTs are connected to each other via contact holes 310.
The device operation of the photoelectric conversion device singly formed will be described by way of example.
FIGS. 15A to 15C are energy band diagrams for explaining the device operation of the photoelectric conversion device shown in FIGS. 13 and 14.
FIGS. 15A to 15C show operations in a refresh mode and in a photoelectric conversion mode, respectively, and show states in the film thickness direction of the layers shown in FIG. 14. A layer M1 is the lower electrode (G-electrode) formed of the first metallic thin film layer (e.g., film of Cr). An a-SiNx layer is an insulating layer which blocks both passage of electrons and passage of holes. It is necessary that the thickness of the a-SiNx layer be large enough to prevent a tunnel effect. Ordinarily, the thickness of the a-SiNx layer is set to 500 angstroms or more. An a-Si-layer is a photoelectric conversion semiconductor layer formed of an intrinsic semiconductor i-layer. An N+-layer is an N-type injection blocking layer for blocking injection of holes into the a-Si-layer. A layer M2 is the upper electrode (D electrode) formed of the second metallic thin film layer (e.g., film of Al).
In the structure shown in FIG. 13, the N+-layer is not completely covered with the D-electrode but the D-electrode and the N+-layer are always equipotential since electrons can move freely therebetween. The following description should be read on this understanding.
This photoelectric conversion device has two operation modes: a refresh mode and a photoelectric conversion mode in correspondence with different ways of applying voltages to the D-electrode and a G-electrode.
In the refresh mode, for example, a negative potential is applied to the D-electrode relative to that applied to the G-electrode, and holes indicated by black round marks in the i-layer are caused by the electric field to move toward the D-electrode, as shown in FIG. 15A. Simultaneously, electrons indicated by white round marks are injected into the i-layer. At this time, part of holes and part of electrons recombine with each other in the N+- and i-layers to disappear. If the device is maintained in this state for a sufficiently long time, holes in the i-layer are swept out from this layer.
To set the device in the mode shown in FIG. 15B from this mode, a positive potential is applied to the D-electrode relative to that applied to the G-electrode. Then electrons in the i-layer are caused to move instantaneously toward the D-electrode. However, holes are not caused to move to the i-layer since the N+-layer functions as an injection blocking layer. When light enters the i-layer in this state, light is absorbed to generate electron-hole pairs. These electrons are caused by the electric field to move toward the D-electrode, while the holes move through the i-layer to reach the interface between the i-layer and the a-SiNx insulating layer. Since the holes cannot move into the insulating layer, they stay in the i-layer. At this time, with the movement of electrons to the D-electrode and the movement of holes to the insulating layer interface of the i-layer, a current flows from the G-electrode to maintain the electrical neutrality in the photoelectric conversion device. This current corresponds to the electron-hole pairs generated by the light and is therefore proportional to the quantity of light entering the photoelectric conversion device. After the device has been maintained for a certain time period in the state in the photoelectric conversion mode shown in FIG. 15B, it enters the state in the refresh mode shown in FIG. 15A. The holes which have stayed in the i-layer are caused to move toward the D-electrode as described above and a current flows which corresponds to this flow of the holes. This amount of holes corresponds to the entire quantity of light entering during the photoelectric conversion mode period. At this time, a current also flows which corresponds to the amount of electrons injected into the i-layer. However, this amount is approximately constant and may be subtracted from the total amount to obtain the detection result. That is, this photoelectric conversion device outputs the quantity of light entering the device in real time, and is also capable of detection of the entire quantity of light entering during a certain period.
However, in a situation where the photoelectric conversion mode period is increased for some reason or in a situation where the illumination intensity of light incident on the device is high, there is a possibility of failure to obtain the desired current while light enters the device. This is because, as shown in FIG. 15C, many holes are accumulated in the i-layer, the electric field in the i-layer is reduced by the holes, and electrons generated are not caused to move but recombined with the holes in the i-layer. This state is called a saturated state of the photoelectric conversion device. If in this state the incident state of light is changed, a current may start to flow unstably in some cases. However, when the photoelectric conversion device is again set in the refresh mode, the holes in the i-layer are swept off. In the following photoelectric conversion mode, a current flows again in proportion to light.
In the operation described above, it is desirable from an idealistic viewpoint that all holes in the i-layer be swept off in the refresh mode. However, sweeping off only part of the holes is effective and sufficient for obtaining the current equal to that described above with no problem. That is, avoiding the state shown in FIG. 15C at the next detection chance in the photoelectric conversion mode may suffice, and the D-electrode potential relative to the G-electrode potential in the refresh mode, the refresh mode period and the characteristic of the injection blocking layer of N+-layer may be determined. Further, injection of electrons into the i-layer in the refresh mode is not a necessary condition and the D-electrode potential relative to the G-electrode potential is not limited to a negative. This is because, in a case where a number of holes are staying in the i-layer, the electric field in the i-layer acts in such a direction as to cause holes to move toward the D-electrode even when the D-electrode potential relative to the G-electrode potential is positive. Also, limitation of the injection blocking layer of N+-layer to such a characteristic that electrons can be injected into the i-layer is not a necessary condition.
FIG. 16 shows a conventional photoelectric conversion circuit corresponding to that for one pixel, which is constituted by a photoelectric conversion device and a TFT.
In FIG. 16, the photoelectric conversion device is represented by a capacitive component Ci formed by the i-layer and a capacitive component CSiN formed by the injection blocking layer. At the point corresponding to the junction between the i-layer and the injection blocking layer (node N in FIG. 16), electrons and holes generated by light recombine with each other when the photoelectric conversion device is saturated, that is, no electric field (only a reduced electric field) exists between the D-electrode and the node N (i-layer). In this state, therefore, hole carriers cannot be stored in the portion N. That is, the node N potential does not exceed the D-electrode potential under any condition. To realize expression of the operation in this saturated state, a diode (D1) is connected in parallel with Ci in FIG. 16. That is, in FIG. 16, the photoelectric conversion device is expressed by three components Ci, CSiN, and D1.
FIG. 17 is a time chart showing the operation of the circuit for one pixel shown in FIG. 16.
The operation of the circuit for one pixel, constituted by the photoelectric conversion device and the TFT, will be described with reference to FIGS. 16 and 17.
A refresh operation will first be described.
It is assumed that Vs is 9V and Vref is 3V. To start the refresh operation, a switch SW-A is set for application of Vref, a switch SW-B is set for application of Vg(on), and a switch SW-C is turned on. In this state, the D-electrode is biased at Vref(6V), the G-electrode is biased at a GND potential, and the node N is biased at Vref(6V) at the maximum. The reason for adding xe2x80x9cat the maximumxe2x80x9d is as described below. If the potential at the node N has already been increased to a level equal to or higher than Vref by the photoelectric conversion operation before the present refresh operation, the node N is biased at Vref via the D1 by the present refresh operation. However, if the potential at the node N resulting from the preceding photoelectric conversion operation is Vref or less, the node N is not biased at the potential Vref by the present refresh operation. It can be said that, in actual use, the node N is biased virtually at Vref(6V) by this refresh operation if the photoelectric conversion operation has been repeated a certain number of times in advance. After the node N has been biased at Ref.(6V), the SW-A is changed to the Vs side. The D-electrode is thereby biased at Vs(9V). By this refresh operation, hole carriers accumulated at the node N of the photoelectric conversion device are swept thoroughly to the D-electrode side.
An X-ray irradiation period will next be described. X-rays are emitted in a pulsating manner, as shown in FIG. 17. A phosphor 1001 is irradiated with X-rays which have passed through a subject. The phosphor loot converts the X-rays into visible light. The visible light from the phosphor is radiated to the semiconductor layer (i-layer) to be converted into an electric signal by photoelectric conversion. Hole carriers produced by photoelectric conversion are accumulated at the node N to increase the potential at this node. Since the TFT is off, the potential at the G-electrode side is correspondingly increased.
A wait period is set between the refresh period and the X-ray irradiation period. In this period, no particular operation is performed and the device is left in a non-operated state to become relieved from a condition of instability in characteristics, if any, due to dark current immediately after refresh operation. If there is no possibility of the photoelectric conversion device being unstable in characteristics immediately after refresh operation, it is not necessary to specially set the wait period.
A transfer operation will next be described. To start the transfer operation, the switch SW-B is set for connection at the Vg(on) side, thereby turning on the TFT. Electron carriers (Se) corresponding to the amount of hole carriers (Sh) accumulated by X-ray irradiation are thereby caused to flow from the C2 side to the G-electrode through the TFT, thereby increasing the potential at the read capacitance C2. The relationship between Se and Sh is as expressed by Se=Shxe2x80x2CSiN/(CSiN+Ci). Simultaneously, the potential at the C2 is amplified by an amplifier to be output. The TFT is maintained in the on state for a time long enough to transfer a sufficient amount of signal charge and is thereafter turned off.
Finally, a reset operation is performed. To start the reset operation, the switch SW-C is turned on. The capacitance C2 is thereby reset to the GND potential, thus preparing for the next transfer operation.
FIG. 18 is a two-dimensional circuit diagram of a conventional photoelectric converter. Only a portion of the converter corresponding to 3xe2x80x23=9 pixels is illustrated for ease of description. Photoelectric conversion devices are indicated by S1-1 to S3-3; switching devices (TFTs) are indicated by T1-1 to T3-3; gate wirings for turning on and off the TFTs are indicated by G1-1 to G3-3; and signal wirings are indicated by M1 to M3. A Vs-line is a wiring for applying a storage bias or refresh bias to the photoelectric conversion devices. Electrodes of the photoelectric conversion devices represented by solid filled rectangles are G-electrodes, while electrodes at the opposite side are D-electrodes. Portions of the Vs-line are used for the D-electrodes. For convenience in enabling entrance of light, a thin N+-layer is used to form the D-electrodes. The group of components: S1-1 to S3-3, T1-1 to T3-3, G1 to G3, M1 to M3, and the Vs-line will be referred to collectively as a photoelectric conversion circuit section (101).
The Vs-line is biased by a Vs power supply 106A or a Vref power supply 106B, which is selected by using a control signal VSC. A shift register 102 is provided to supply a drive pulse voltage to the wirings G1 to G3. The voltage for turning on the TFTs is determined by and externally supplied from a power supply (VG(on)). A read circuit section 107 is provided in which parallel signal outputs from the wirings M1 to M3 in the photoelectric conversion circuit section 101 are amplified and converted into a serial signal to be output.
In the read circuit section 107 are provided switches RES1 to RES3 for resetting M1 to M3, amplifiers A1 to A3 for amplifying signals from M1 to M3, sample and hold capacitors CL1 to CL3 for temporarily storing the signals amplified by the amplifiers A1 to A3, switches Sn1 to Sn3 for sampling and holding, buffer amplifiers B1 to B3, switches Sr1 to Sr3 for converting parallel signals into a serial signal, a shift register 103 for supplying pulses for parallel-to-serial conversion to the switches Sr1 to Sr3, and a buffer amplifier 104 for outputting the converted serial signal.
FIG. 19 is a time chart showing the operation of the photoelectric converter shown in FIG. 18. The operation of the photoelectric converter shown in FIG. 18 will be described with reference to the time chart of FIG. 19. Through control signal VSC, one of two different biases is applied to the Vs-line of the photoelectric conversion devices, i.e., the D-electrodes of the photoelectric conversion devices. Each D-electrode has a potential Vref (V) when the VSC is xe2x80x9cHixe2x80x9d and a potential VS(V) when the VSC is xe2x80x9cLoxe2x80x9d. DC power supplies 106A and 106B supply the read power supply voltage VS(V) and refresh power supply voltage Vref (V), respectively.
The operation in a refresh period will be described. All the signals from the shift register 102 are set to xe2x80x9cHixe2x80x9d and a signal CRES in the read circuit section is set to xe2x80x9cHixe2x80x9d. All the switching TFTs (T1-1 to T3-3) are thereby turned on, the switching devices RES1 to RES3 in the read circuit section are also turned on, and the GND potential is set at the G-electrodes of all the photoelectric conversion devices. When the signal VSC becomes xe2x80x9cHixe2x80x9d, the D-electrodes of all the photoelectric conversion devices are biased at the refresh power supply voltage Vref (negative potential). All the photoelectric conversion devices S1-1 to S3-3 are thereby set in the refresh mode, thus performing refreshment.
The operation in a photoelectric conversion period will next be described. The VSC is changed to xe2x80x9cLoxe2x80x9d and the D-electrodes of all the photoelectric conversion devices are biased at the read power supply voltage VS (positive potential). The photoelectric conversion devices are thereby set in the photoelectric conversion mode. In this state, all the signals from the shift registers 102 are set to xe2x80x9cLoxe2x80x9d and the signal CRES in the read circuit section is also set to xe2x80x9cLoxe2x80x9d. All the switching TFTs (T1-1 to T3-3) are thereby turned off and the switching devices RES1 to RES3 in the read circuit section are also turned off. The G-electrodes of all the photoelectric conversion devices are set in a DC-open state. However, the potential at each G-electrode is maintained since the photoelectric conversion device also functions as a capacitor. At this point of time, however, no light enters each photoelectric conversion device and no charge is generated in the photoelectric conversion device, that is, no current flows. When in this state a light source is turned on in a pulsating manner, the D-electrode (N+-electrode) of each photoelectric conversion device is irradiated with light and a so-called photocurrent flows through the device. The light source, although not shown in FIG. 18, is, for example, a fluorescent lamp, LEDs, or a halogen lamp in the case of use in a copying machine. In the case of use in an X-ray image pickup apparatus, the light source is, of course, an X-ray source. In this case, a scintillator for conversion of X-rays into visible light may be used. Photocurrent caused by light to flow accumulates as charge in each photoelectric conversion device, which is held after the light source has been turned off.
Next, the operation in a read period will be described. The read operation is performed in the order of the first line corresponding to S1-1 to S1-3, the second line corresponding to S2-1 to S2-3, and the third line corresponding to S3-1 to S3-3. First, to read first-line signals from S1-1 to S1-3, the shift register SR1 supplies a gate pulse to the gate wiring G1 connected to the switching devices (TFTs) T1-1 to T1-3. The high level of the gate pulse supplied at this time is a voltage Vcom externally supplied. The TFTs T1-1 to T1-3 are thereby turned on and signal charges accumulated in S1-1 to S1-3 are transferred to the signal wirings M1 to M3. Read capacitances (not shown in FIG. 18) are added to the signal wirings M1 to M3 and the signal charges are transferred to the read capacitances via the TFTs. For example, the read capacitance added to the signal wiring M1 is the sum of the gate-source interelectrode capacitances (Cgs) (three capacitances) of the TFTs T1-1, T2-1, and T3-1 connected to the signal wiring M1, and corresponds to C2 shown in FIG. 16. The signal charges transferred to the signal wirings M1 to M3 are amplified by the amplifiers A1 to A3. Then the signal CRES is turned on to transfer the charges to the sample and hole capacitors CL1 to CL3. When the signal CRES is turned off, the transferred charges are held on the capacitors CL1 to CL3. Subsequently, the shift register 103 applies a pulse to the switches Sr1, Sr2, and Sr3 so that the switches receive the pulse one after another in the order of Sr1, Sr2, and Sr3. The signals held on the capacitors CL1 to CL3 are then output from the amplifier 104 in the order of CL1, CL2, and CL3. Consequently, the signals obtained by photoelectric conversion from S1-1, S1-2, and to S1-3 for the first line are successively output. The operation for reading second-line signals from S2-1 to S2-3, and the operation for reading third-line signals from S3-1 to S3-3 are performed in the same manner.
If signals on the wirings M1 to M3 are sampled and held on CL1 to CL3 by using the first-line sample signals, then the wirings M1 to M3 can be reset to the GND potential by the signal CRES, followed by application of the gate pulse to the wiring G2. That is, while parallel-to-serial conversion of the first-line signals is performed by the shift register SR2, second-line signal charges from the photoelectric conversion devices S2-1 to S2-3 can be simultaneously transferred by the shift register SR1.
The signal charges on all the photoelectric conversion devices corresponding to the first to third lines can be output by the above-described read operations.
To obtain a series of moving images, the operation shown in the time chart of FIG. 19 may be repeated the number of times corresponding to the number of moving images to be obtained.
However, to obtain moving images having an increased number of pixels, it is necessary to further improve the frame frequency.
In case that a refreshment operation is performed through Vs line common to all photoelectric conversion devices, it would be necessary to provide one refreshment period per one frame. That would cause a problem that, at a movie image pick-up, frame frequency would be made small, that is, speed would be slower.
In general, design as a specification for simple image pick-up of a chest, it is required that an image pick-up area is not smaller than 40 cm square, pixel pitch is not larger than 200 xcexcm. For example, when the image pick-up area is 40 cm square and the pixel pitch is 200 xcexcm, total number of the photoelectric conversion devices is 4,000,000. When such larger number of image pick-up devices are to be refreshed collectively simultaneously, a larger current flows at the refreshment. Accordingly, a voltage change at GND and a power source line of X-ray image pick-up apparatus would be larger. In image pick-up of a particular case, during a time period of making stable the voltage change, it would be necessary to provide waiting period before the X-ray irradiation. That is the WAIT period shown in FIG. 17. That is, for the simultaneous and collective refreshment of the photoelectric conversion devices, it is necessary not only to provide the one refreshment period in one frame, but also to provide one wait period in one frame.
As described in the above, it is a technical problem in the prior art that one refreshment at all image pick-up devices would be necessary for one reading operation, and thus the movie image pick-up would be difficult.
The present invention has been made in view of the above-mentioned problems, and in order to solve the problems, according to the present invention, there is provided a radiographic image pickup apparatus comprising: a substrate; a plurality of pixels each of which is constituted by an MIS-type photoelectric conversion device and a switching device, the pixels being two-dimensionally arranged on the substrate; a plurality of control wirings connected to control electrodes of the switching devices; a plurality of signal wirings for reading signals from the MIS-type photoelectric conversion devices; and second switching means for switching a bias for turning on the switching device to at least one of a first bias and a second bias.
Detailed description thereof will be made with regard to the following embodiments of the present invention.